There is a bit of documentation provided by Wu Xiancheng. See his comment in the SessionHandlerInterface section:
https://www.php.net/manual/en/class.sessionhandlerinterface.php#122032
(PHP 7, PHP 8)
SessionUpdateTimestampHandlerInterface is an interface which defines optional methods for creating a custom session handler. In order to pass a custom session handler to session_set_save_handler() using its OOP invocation, the class can implement this interface.
Note that the callback methods of classes implementing this interface are designed to be called internally by PHP and are not meant to be called from user-space code.
There is a bit of documentation provided by Wu Xiancheng. See his comment in the SessionHandlerInterface section:
https://www.php.net/manual/en/class.sessionhandlerinterface.php#122032